they are two different mediums. They tend not to be used together. Graphite is somewhat metallic and reflective. Because of this charcoal wont hold very well on thicker layers of it. It is also harder and easier to score the paper with which can be seen when you layer a coat of charcoal over top.
For preliminary sketches of longer charcoal pieces I would advise soft vine charcoal sticks or HB charcoal pencils applied with the lightest of touches. you can sharpen them to a long tapered point so you have more control over the quality of your sketch lines.
I really don't see any problem with starting out in basic pencil before hitting it with charcoal. If your using the pencil to do some basic contour lines to get the shapes nailed out then it won't really be an issue if you use pencil, vine charcoal or a charcoal pencil. But if you go with pencil it doesn't matter how heavy handed you may be in that the lines will dissapear under the charcol once its laid out.
One of my favorite techniques is to rub the paper with some charcoal then smear it with a rag of some sort creating a nice medium value and then taking an eraser and doing a quick structural layout that way.
The best thing for you to do is actually try out all these methods and see what works best for you.
Speaking purely from my personal experience, I don't like sketching with pencil under charcoal. Unless you have a really light hand (which I don't) the pencil will leave score marks in the paper. I just found it distracting. Still, it's worth trying since you may like it. I'd advise a very soft pencil, like ebony, and light pressure.
Actually, in general I'd advise trying just about anything you can think of technique-wise... you may find a combination that really works for you.
Forgot to mention... I second the call for vine as being the nicest general-purpose charcoal.
"Change is a virtue my friend... if you want to escape, all you have to do is make up your mind."
John Cale / Bob Neuwirth
With good paper, and a putty rubber, charcoal (unless its the deathly awful compressed stuff) can be moved around pretty indefinately. Meaning you can place in tones, change them, move them, erase them at your hearts content.
This should lead to the pencil start being completely unneccesary. This allows your work to flow much more simply, starting and finishing with charcaol.